M.A. Nagarajan

Optical model analyses of 6,7Li + 208Pb elastic scattering near the Coulomb barrier

The elastic scattering of 6Li and 7Li by 208Pb has been measured at several energies near the Coulomb barrier and analysed in terms of the optical model. Previous reports have suggested that the normalization factor, NR, for the double-folded real potential is approximately 0.6 for both isotopes at bombarding energies well above the Coulomb barrier. In this paper we show that, whereas this low value for NR persists for 6Li at energies close to the barrier, for 7Li it is somewhat greater than unity, and indicates the existence of a threshold anomaly for this projectile.

The threshold anomaly in 16O + 208Pb scattering

Abstract The empirically observed energy dependence of the optical potential at energies close to the Coulomb barrier (the threshold anomaly) in the collisions of 16 O on 208 Pb is investigated within the framework of coupled-reactions-channels (CRC) calculations. The motivation is to determine the extent to which the observed threshold anomaly can be accounted for by strong coupling of the elastic to the inelastic and transfer channels. CRC calculations have been performed at a range of energies above and below the Coulomb barrier, both for the complete set of reaction channels, and also, to see the effects more simply, for individual inelastic and transfer channels. An energy-dependent local polarisation potential has been extracted. It will be seen how this polarisation potential satisfies the dispersion relations, and how well it reproduces the elastic scattering and the fusion cross sections over a range of energies.

Heavy-ion fusion: Channel-coupling effects, the barrier penetration model, and the threshold anomaly for heavy-ion potentials*

We study the formal structure of the influence of channel coupling on near- and sub-barrier fusion. The reduction to a one-channel description is studied, with emphasis on the channel-coupling effects being manifest primarily as an energy dependence (the “threshold anomaly”) of the real nuclear potential. The relation to the barrier penetration model is examined critically. The results of large-scale coupled-channel calculations are used as “data” to illustrate the discussion. Particular emphasis is placed on the importance of reproducing correctly the partial-wave (or “spin”) distributions. The simple barrier penetration model is found to be adequate to exhibit the strong enhancements due to channel couplings when the threshold anomaly is taken into account, although there may be important corrections due to the long-ranged peripheral absorption, especially from Coulomb excitation.

Energy dependence of the 16O + 60Ni potential and the optical model dispersion relation

Abstract A systematic analysis of 16 O + 60 Ni elastic scattering data over a range of bombarding energies reveals strong variations of both the real and imaginary parts of the optical potential near the Coulomb barrier. The results are consistent with a dispersion relation interpretation of the effect which predicts a peak in the real surface potential close to the barrier.

The threshold anomaly in the 16O + 58,60,62,64Ni systems

Abstract Accurate angular distributions of 16 O ions scattered by 58,60,62,64 Ni have been measured at five energies between 60 and 120 MeV. An optical model analysis of these, together with existing data, indicates that, whereas there is a significant increase with A in the surface absorption, there is little, if any, isotopic dependence in the energy variation of the optical potentials, V ( E ) and W ( E ), near threshold (the threshold anomaly). Clear evidence is found for a hitherto unreported peak in W ( E ) just above threshold. CRC calculations, including inelastic excitation and transfer, reproduce the data but show no clear connection between the peak and a particular reaction channel.

Cranked-shell-model calculations in a truncated space

Abstract Cranking calculations are presented for a deformed single-j shell containing particles which interact through short-range two-body forces. Truncation schemes which might pave a way for carrying out realistic multi-j shell calculations are investigated.

Neutron flow and necking in heavy-ion fusion reactions

Abstract The “barrier distributions” arising from neutron transfer channels are discussed. It is shown that sequential transfers can lead to the broad distributions characteristic of many experimental fusion cross sections. Finite Q-value effects can lead to neutron flow and a build up of a neck between the colliding nuclei. “Anti-necking” may also occur.

Multistep effects in reactions of 17O with 208Pb near the coulomb barrier

Abstract The elastic scattering, inelastic scattering and single-neutron transfer cross sections in the collision of 78 MeV 17 O on 208 Pb are measured. It is shown that these cross sections can be understood in terms of a three-body model of the system comprising a 16 O core, a valence neutron and a 208 Pb target. The theoretical analysis points to the importance of multistep effects in all reaction processes. The inherent energy dependence of the 16 O 208 Pb elastic optical potential at energies close to the Coulomb barrier is also seen in 17 O 208 Pb elastic scattering. The effect of the valence-neutron channel coupling on the 17 O 208 Pb interaction potential is deduced, and is shown to be almost entirely imaginary. Implications for a possible isotope dependence of the sub-barrier fusion cross section are considered.

Effect of deformation on the elastic and quasielastic scattering of heavy ions near the Coulomb barrier

Abstract It is shown that elastic and quasielastic cross sections at large angles provide the same information on static deformation properties as does sub-barrier fusion. The results have their “classical” form even for coupling to a small number of excited states.

Effect of multiphonon coupling on heavy-ion fusion

Abstract The effect of the strong coupling of multiphonon states on the fusion of heavy ions is discussed. The model (CCFUS) of Dasso and Landowne is reviewed and it is shown that it is possible to generalise this to multiphonon states comprised of identical phonons without greatly complicating the calculations. The effect of the muliphonon channels on the barrier distribution for fusion is discussed in detail. In particular, it is shown that for such a system, one barrier may be pulsed up to rather high energies. This high-energy barrier will be difficult to see in the fusion data but we show that it may have a significant effect on the large-angle elastic (or quasielastic) excitation functions.